A known technique for controlling the temperature of a heating and/or cooling region is to cyclically activate a compressor motor to force refrigerant through a heat exchanger in thermal communication with the region. There are disadvantages in operating a compressor motor in this cyclic on/off fashion. The compressor motor's life is shortened when it cycles on and off and such operation is not the most energy efficient heat exchange method. If the on/off durations are lengthened (using a hysteresis control for example) less precise control over the temperature within the region occurs since the temperature continually cycles above and below the desired set-point temperature.
U.S. Pat. No. 4,893,479 to Gillett which issued Jan. 16, 1990 and is assigned to the assignee of the present invention discloses a variable speed motor control for a multi-phase induction motor. The disclosure of this patent is incorporated herein by reference.
The motor control disclosed in the '479 patent activates a three-phase induction motor at a continuously variable speed by controlled activation of an inverter circuit. The motor control disclosed in the '479 patent is described as being for use with any multi-phase compressor motor. The refrigeration uses of the '479 system thus include refrigerators, freezers, heat pumps, or air conditioners. Instead of operating the compressor motor at a constant speed during an on cycle, the system disclosed in the '479 patent activates the compressor motor at a continuously variable range of speeds during heating or cooling periods. The heating or cooling demands of the control volume dictate the speed at which the motor operates. So long as the heating and/or cooling demands do not change, the motor control causes the compressor motor to operate at a narrow range of speed which satisfies the heating and/or cooling requirements and is not subject to the cyclic temperature variations caused by intermittent compressor motor operation.
The apparatus disclosed in the '479 patent includes a circuit for generating a speed control signal and a voltage control oscillator for generating a cyclicly varying wave form having a pulse width modulated output to achieve constant motor torque. The constant torque output of the induction motor disclosed in the '479 patent is achieved by maintaining the volts per hertz across the motor winding constant regardless of motor speed. While the system disclosed in the '479 patent achieves this function, the step voltage increments used to simulate a standard sinusoidal energization of the motor windings are only approximations.
The application of a constant volts per hertz to provide uniform torque output from a variable speed multi-phase motor is a principal goal of the motor control of the '479 patent. The present invention, achieves multi-phase variable speed motor operation in response to a control input utilizing the general principals disclosed in the '479 patent. By extending these principles, however, a more accurate and flexible motor energization control is achieved.